Transparent resins have heretofore been used as ordinary materials for automobile parts, lighting equipments, electric parts, etc., of which transparency is required, and have recently been being applied to, particularly, optical materials that optical properties are of great importance. Polycarbonate resins and acrylic resins are known as transparent resins suitably used for such application fields.
However, the acrylic resins involve problems in points of heat resistance and water resistance (low water absorption property) and the like though they are excellent in transparency. On the other hand, the polycarbonate resins involve such problems that birefringence is high though they are superb in heat resistance and water resistance to the acrylic resins. Therefore, as transparent resins for optical materials, attention has been recently attracted to cycloolefin resins that have good transparency, water resistance, low birefringence, heat resistance and the like in combination and is supplied to actual use.
As such cycloolefin reins, various resins have been known and described in, for example, Patent Literature 1 to Patent Literature 6.
With the high advancement of function and use of optical instrument, however, these cycloolefin resins may not sufficiently satisfy properties required of optical materials in some cases. In optical materials used in optical instrument having highly advanced functions in particular, sufficiently low birefringence is extremely important property. There is thus a demand for development of a transparent resin having far excellent low birefringence compared with the conventional transparent resins.
On the other hand, attention is attracted to cycloolefin resins as resin materials used in optical films. Films formed of such a cycloolefin resin have been proposed as various optical films.
Specifically, phase difference plates formed of a film of a cycloolefin resin are described in Patent Literature 7 to Patent Literature 10.
In Patent Literature 11 to Patent Literature 13, it is described that a film of a cycloolefin resin is used as a protective film for a polarizing plate.
A substrate for a liquid crystal display device, which is composed of a film of a cycloolefin resin, is described in Patent Literature 14.
Since the optical films formed respectively of these cycloolefin reins have property that the absolute value of phase difference (birefringence) of transmitted light becomes lower as the wavelength of the transmitted light becomes longer, however, it is extremely difficult to give a specific phase difference of, for example, a ¼ wavelength to the transmitted light of all rays in a visible region (400 to 800 nm). The same circumstances apply to optical films formed respectively of other resins in addition to the conventional cycloolefin resins.
As described above, in resins used in application fields of which highly-advanced optical properties are required, other than a fact that they have general optical properties, the control of specific wavelength dependency about birefringence that the birefringence of transmitted light becomes higher as the wavelength of the transmitted light becomes longer when a film is formed, and degree of the birefringence become great problems.
Prior Art 1: Japanese Patent Application Laid-Open No. 132625/1989;
Prior Art 2: Japanese Patent Application Laid-Open No. 132626/1989;
Prior Art 3: Japanese Patent Application Laid-Open No. 133413/1990;
Prior Art 4: Japanese Patent Application Laid-Open No. 120816/1986;
Prior Art 5: Japanese Patent Application Laid-Open No. 115912/1986;
Prior Art 6: Japanese Patent Application Laid-Open No. 218726/1988;
Prior Art 7: Japanese Patent Application Laid-Open No. 245202/1992;
Prior Art 8: Japanese Patent Application Laid-Open No. 36120/1992;
Prior Art 9: Japanese Patent Application Laid-Open No. 2108/1993;
Prior Art 10: Japanese Patent Application Laid-Open No. 64865/1993;
Prior Art 11: Japanese Patent Application Laid-Open No. 212828/1993;
Prior Art 12: Japanese Patent Application Laid-Open No. 51117/1994;
Prior Art 13: Japanese Patent Application Laid-Open No. 77608/1995;
Prior Art 14: Japanese Patent Application Laid-Open No. 61026/1993;